Along with the development of an on-vehicle communication system and the gradual maturation of the technology of mobile ad-hoc networking, the Dedicated Short Range Communication (DSRC) protocol for the car networking has been developed internationally especially for real-time, dynamic and intelligent management of vehicles. With the DSRC protocol, information is transmitted bi-directionally to organically link one vehicle to another and the vehicles to information acquisition devices on a roadside so as to support point-to-point and point-to-multipoint communication.
The Mobile Slotted ALOHA (MS-ALOHA) mechanism is a time-division based DSRC Medium Access Control (MAC) layer access and resource allocation mechanism where resources are allocated in a unit of slot based on the frame structure. Referring to FIG. 1, one frame includes N slots numbered 0 to N−1, and these numbers are cyclically repeated from one frame to another. Only one vehicle is allowed to transmit in each slot, that is, there is a Time Division Multiple Access (TDMA) mode between the vehicles. The vehicle transmits both application layer data and Frame Information (FI) in the occupied slot, where occupancy states of respective slots in one frame are indicated in the FI, and FIG. 2 illustrates a possible structure of the FI, for example.
A general idea of the MS-ALOHA mechanism lies in that any node (e.g., a vehicle) accessing the network needs to occupy one slot by monitoring idle slot resources in the frame, and if the node does not give up actively the occupied slot resource, then the node can transmit data in the occupied slot all the time, and the slot will be inaccessible to the other nodes throughout this period of time. The node needs to transmit periodically the FI in the occupied slot, where the FI carries information of conditions of occupying slot, obtained by the node, of the other nodes, within two hops from the node, to indicate occupancy condition information of each slot perceived by the node (also referred to slot state information or slot information), where for each slot, the following information of the slot is indicated: slot occupancy state information, a Source Temporary Identifier (STI) corresponding to a node occupying the slot, referred to as a node identifier, and a priority state of the node occupying the slot (or a priority state corresponding to data transmitted in the slot by the node occupying the slot), where the slot occupancy state information can represent four occupancy states of the slot: (00) represents that the slot is idle, (10) represents that the slot has been occupied by another node at one hop from the current node (simply referred to as a one-hop node) or by the current node, (11) represents that the slot has been occupied by another node at two hops from the current node (simply referred to as a two-hop node), and (01) represents that the slot has been occupied by more than two other nodes, i.e., a collision state; and each node can monitor in a slot, which is not occupied by the node, FI transmitted by an adjacent one-hop node to thereby determine a condition of occupying slot of each adjacent node within a range of three hops, and request a new idle slot again upon detecting that the slot resource occupied by the current node collides with a resource accessed by another node. For the sake of a convenient description later, the FI and the information contents therein will be described as follows throughout this application:
Frame Information (FI) transmitted by a node will be referred to as an FI message or simply FI;
Occupancy condition information corresponding to each slot indicated in the FI will be referred to as a slot information field corresponding to each slot in the FI message; and
Three kinds of information indicated in the occupancy condition information corresponding to each slot in the FI (i.e., slot occupancy state, STI and priority information) will be referred to respectively as a slot occupancy state sub-field, a STI sub-field and a priority sub-field included in the slot information field of each slot.
It shall be noted that the FI and the information contents therein have been described above only for the sake of a convenient description later, or of course, they can alternatively be described otherwise.
Under the MS-ALOHA mechanism, the node needs to maintain the occupied slot by maintaining a slot state buffer table with (N−1)*N elements, storing the slot information fields of the respective slots carried in the FI message, transmitted by the adjacent node, received in the corresponding slot. Referring to FIG. 3, for example, there is illustrated an N*N-element slot state buffer table, and since the FI message transmitted by the node in the occupied slot does not need to be stored, the slot state buffer table really maintained by the node includes N−1 rows (on the assumption that each node occupies only one slot), and the (N−1)*N slot state buffer table to be described later in this application will relate to a table in which the slot information of the FI transmitted by the node in the occupied slot is not stored; where a detection field corresponding to a slot refers to a slot information field corresponding to the occupied slot in the FI message transmitted in the slot, referred to as a “detection field” of the slot, and a “non-detection field” refers to a slot information field corresponding to the slot in the FI transmitted in another slot than the occupied slot, referred to as a “non-detection field” of the slot; and a default value is a value by default.
Upon reception of an FI message in a slot, the node always overwrites the information contents in the row, where the corresponding slot is located, in the slot state buffer table (i.e., the contents recorded for the last frame periodicity) with slot information contents carried in the newly received FI message, particularly as follows:
The node generates and transmits an FI message in a slot occupied by the node, where the respective fields, including slot occupancy state sub-fields, STI sub-fields and priority sub-fields, needs to be filled in under some rule. The node will clear the transmitted FI information after transmitting the FI message.
The node needs to receive in a slot which is not occupied by the node an FI message transmitted by an adjacent node, updates the slot state buffer table according to the received FI message, and before the node proceeds to the slot occupied by the current node, judges whether the slot occupied by the node has been maintained successfully, and determines occupancy states of respective slots other than the slot occupied by the node, where if there is no FI received in another slot than the slot occupied by the node, then the node will fill the default value in the respective fields of the row corresponding to the slot in the slot state buffer table. The default value is currently defined as the idle state (00), or of course, the default value can alternatively be defined otherwise.
At present the existing slot resource allocation mechanism based upon the exchanged FI is generally focused on the FI being exchanged so that the respective nodes perceive the slot occupancy states of the adjacent nodes to thereby determine the slot resources which can be requested by the respective nodes. However, in the car networking, there is a strict requirement on a transmission delay of a message in traffic safety and traveling efficiency related applications, for example, the transmission delay of a message is generally required not to exceed 100 ms, but such a required transmission delay in these application may be difficult to accommodate in the traditional cellular network where resources are centrally allocated, for example, if the transmission delay is required to be “a delay of 100 ms from the current time”, then the transmitting of the message may tend to be delayed because it is very likely for a traditionally requested slot resource to become accessible after 100 ms.
As well known, an unredeemable aftereffect may occur as a result of a transmission delay of a message in an application with a strict requirement on the transmission delay, so there is a need of redesigning a slot resource scheduling mechanism to accommodate a low delay required in exchanging a message over the car networking.